1. Field of the Invention
The present invention relates to a producing technique for a material of bearings incorporated in internal combustion engines for automobiles, marine vessels, agricultural machines, construction machines, etc. and of sliding bearings used in rotating or reciprocating sliding parts of general machines, and more particularly to an aluminum-base composite bearing material excellent in wear resistance property and anti-seizure property, and a method of producing the same.
2. Description of the Related Art
A conventional aluminum alloy bearing with a back metal is produced through a casting or a sintering process.
In the casting process, an aluminum alloy is continuously cast to form a strip material having a thickness of 15 to 20 mm and a constant width. The strip material is rolled and annealed repeatedly to have a thickness of about 1 mm. Thereafter, the strip material is integrally bonded to a steel strip by rolling to produce a bearing material. It is noted that, according to the casting process, in the case where Pb, a solid lubricant, which is not soluble in aluminum, is added to an aluminum alloy in an amount of not less than 5%, a melt of the aluminum alloy separates into two phases unless the casting temperature is not lower than 1000xc2x0 C. While this is one example, in casting, process conditions considerably vary depending on additive alloy components, and in some cases, there is a case that a production is impossible.
On the other hand, in the production of an aluminum-base composite bearing material by the powder metallurgical process, a continuous strip material consisting of a bonding layer, an alloy layer and a sacrificial layer is formed by means of a rolling machine for powder compacting, and after sintering, the material is integrally bonded to a steel strip as a back metal by rolling as disclosed in JP-B2-49-45445 and JP-B2-2502600.
According to another example of the powder metallurgical process as disclosed in JP-A-52-20336, there is proposed a method that an aluminum-base powder is directly spread on a back metal followed by rolling, sintering and again rolling.
The present invention relates to an improvement in the method for producing an aluminum-base sliding bearing material by the above powder metallurgical process.
Recently, there have been strong desires of performance improvement to sliding bearings used in rotating or reciprocating sliding parts in passenger cars and the like, that are long durability, maintenance free property and so on. This is a demand for a high quality material. The technology intended to be improved by the invention is a powder metallurgical method according to which an aluminum-base composite strip, having a three layers structure consisting of a bonding layer, a sliding layer and a sacrificial layer, is formed beforehand, and after sintering, both side edge sections thereof are trimmed to provide the sintered product with a predetermined width size so as to be in conformity with the width of a steel strip (back metal) to which the sintered product is bonded, and finally the sintered product is bonded to the steel strip by rolling.
In the methods disclosed in JP-B2-49-45445 and JP-B2-2502600, the moisture on the surface of each particle of the powder or various aluminum hydroxides vaporize to expand with temperature rising when sintering the powder. Most of the generated gas is released to the outside through zones of a molten low-melting-point metal as pathways in the sacrificial layer (surface layer) containing the dispersed low-melting-point metal. However, a part of the gas remains without release to produce blisters in the material, where the low-melting-point metal moves. Even after the rolling-bonding of the sintered material to the steel strip in a subsequent process, there has been often observed a phenomenon that collapsed blisters under a rolling-bonding pressure become material defects due to segregation of the low-melting-point metal at the collapsed blisters. In particular, a number of such material defects occur in spring and summer seasons characterized by high temperature and high humidity which are weather characteristics of Japan.
In a sliding bearing with such material defects (i.e. blisters), contrary to the required bearing performance, when using a bearing, there have been often occurred unfavorable accidents such as delamination of material, abnormal wear and seizure at blisters. Further, in the production process for bearings, it is necessary to remove material defects detectable from an appearance, so that an improvement in productivity is excessively prevented.
The invention has been proposed under the above background.
In view of the fact that, when heating and sintering the above mentioned aluminum-base composite strip having a three layers structure consisting of the bonding layer, the sliding layer and the sacrificial layer, gases such as H2O, H2, and O2 are generated in the aluminum-base composite strip, a primary object of the invention is to restrain occurrence of blisters in the strip by allowing the gases generated in the strip to easily release not only from the sacrificial layer but also from the bonding layer whereby improving the quality of the product strip.
Under the object, the present inventors have proposed the following method for producing an aluminum-base composite bearing material made by powder sintering.
The objective aluminum-base composite bearing material made by powder sintering consists of an aluminum-base metallic layer made by powder sintering and a back metal, the aluminum-base metallic layer consisting of a bonding layer, a sliding layer and a sacrificial layer, wherein the sintered aluminum-base powder layer is bonded by rolling to the back metal at the side of the bonding layer whereby the back metal, the bonding layer, the sliding layer and the sacrificial layer are arranged in this order.
According to the invention, the aluminum-base composite bearing material made by powder sintering is produced through the following eight steps:
(a) preparing a first powder to be formed to the bonding layer, which consists of 0.4 to 5 mass % in total of at least one selected from a first element group of Sn, In and Pb, from zero to 2 mass % in total of at least one selected from a second element group of Si, Mn, Ni, Cr, Zn, Fe, Zr, Ti and Sb, from zero to 1 mass % in total of at least one selected from a third element group of Cu and Mg, and the balance of Al and incidental impurities;
(b) preparing a second powder to be formed to the sliding layer, which consists of 2 to 20 mass % in total of at least one selected from the first element group, 2 to 8 mass % in total of at least one selected from the second element group, 0.5 to 2 mass % in total of at least one selected from the third element group, and the balance of Al and incidental impurities;
(c) preparing a third powder to be formed to the sacrificial layer, which consists of 0.4 to 25 mass % in total of at least one selected from the first element group, from zero to 4 mass % in total of at least one selected from the second element group, from zero to 1 mass % in total of at least one selected from the third element group, and the balance of Al and incidental impurities;
(d) continuously forming an elongated multi-layered body of three powder layers so that the second powder is positioned between the first and second powders, whereby obtaining a first to third powder layers;
(e) continuously forming a composite strip of compacted powders, which has a three layers structure, by continuously feeding the elongated multi-layered body into a rolling machine for powder compacting to press and form it;
(f) producing a sintered composite strip by heating the continuous composite strip of compacted powders to a temperature of 460xc2x0 C. to 550xc2x0 C. to continuously sinter it;
(g) continuously laying the sintered composite strip on a steel strip to be the back metal, which is fed separately, so as to bring the first powder layer side of the sintered composite strip to contact with the steel strip, and feeding the combined strips into a rolling machine to continuously rolling-bond the sintered composite strip to the steel strip; and
(h) subjecting the thus obtained rolling-bonded composite strip to a heat treatment of heating it to a temperature of 400xc2x0 C. to 510xc2x0 C. and holding it under the temperature for not less than 30 seconds, and of rapidly cooling it down to 300xc2x0 C. at a cooling rate of not lower than 50xc2x0 C./minute.
By the above steps, a multi-layered composite material having a layered structure can be obtained, which consists of the back metal of the steel strip, the bonding layer of the sintered first powder layer, the sliding layer of the sintered second powder layer and the sacrificial layer of the sintered third powder layer, these being arranged in this order.
It is preferable that the rolling-bonded composite strip is subjected to a heat treatment at a temperature of 250xc2x0 C. to 400xc2x0 C. prior to the step (h).
A preferred embodiment of heat treatment conditions for the continuous composite strip of compacted powders in the step (f) is as follows:
the temperature-elevating rate to a temperature of 460xc2x0 C. to 550xc2x0 C.: 20xc2x0 C. to 50xc2x0 C./hour, and
the holding time at the elevated temperature: 2 to 16 hours.
Further, according to a preferred embodiment, the composite strip having a three layers structure obtained in the step (e) has a total thickness of 1200 to 2500 xcexcm, a thickness of the first powder layer to be formed to the bonding layer of 300 to 600 xcexcm, and a thickness of the third powder layer to be formed to the sacrificial layer of 100 to 300 xcexcm.
According to a second aspect of the invention, there is provided an aluminum-base composite bearing material consisting of an aluminum-base metallic layer made by powder sintering and a back metal, wherein the aluminum-base metallic layer consists of a bonding layer, a sliding layer and a sacrificial layer which are arranged in this order, and wherein the aluminum-base metallic layer is bonded by rolling to a steel strip to be the back metal at the side of the bonding layer. The aluminum-base composite bearing material is characterized by that:
(a) the bonding layer is a sintered powder layer consisting of 0.4 to 5 mass % in total of at least one selected from a first element group of Sn, In and Pb, from zero to 2 mass % in total of at least one selected from a second element group of Si, Mn, Ni, Cr, Zn, Fe, Zr, Ti and Sb, from zero to 1 mass % in total of at least one selected from a third element group of Cu and Mg, and the balance of Al and incidental impurities;
(b) the sliding layer is a sintered powder layer consisting of 2 to 20 mass % in total of at least one selected from the first element group, 2 to 8 mass % in total of at least one selected from the second element group, 0.5 to 2 mass % in total of at least one selected from the third element group, and the balance of Al and incidental impurities;
(c) the sacrificial layer is a sintered powder layer consisting of 0.4 to 25 mass % in total of at least one selected from the first element group, from zero to 4 mass % in total of at least one selected from the second element group, from zero to 1 mass % in total of at least one selected from the third element group, and the balance of Al and incidental impurities; and
(d) the aluminum-base metallic layer consisting of the bonding layer, the sliding layer and the sacrificial layer is a sintered composite strip which is formed prior to the rolling-bonding to the steel strip, and the aluminum-base composite bearing material is, as a whole, subjected to a heat treatment after the sintered composite strip is bonded by rolling to the steel strip.
According to a third aspect of the invention, there is provided an aluminum-base composite bearing material, wherein the sacrificial layer of the above aluminum-base composite bearing material is removed by machining.